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From Fins Into Hands: Scientists Discover a Deep Evolutionary Link

To help his readers fathom evolution, Charles Darwin asked them to consider their own hands.

“What can be more curious,” he asked, “than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include similar bones, in the same relative positions?”

Darwin had a straightforward explanation: People, moles, horses, porpoises and bats all shared a common ancestor that grew limbs with digits. Its descendants evolved different kinds of limbs adapted for different tasks. But they never lost the anatomical similarities that revealed their kinship.

As a Victorian naturalist, Darwin was limited in the similarities he could find. The most sophisticated equipment he could use for the task was a crude microscope. Today, scientists are carrying on his work with new biological tools. They are uncovering deep similarities that have been overlooked until now.

On Wednesday, a team of researchers at the University of Chicago reported that our hands share a deep evolutionary connection not only to bat wings or horse hooves, but also to fish fins.

The unexpected discovery will help researchers understand how our own ancestors left the water, transforming fins into limbs that they could use to move around on land.

To the naked eye, there is not much similarity between a human hand and the fin of, say, a goldfish. A human hand is at the end of an arm. It has bones that develop from cartilage and contain blood vessels. This type of tissue is called endochondral bone.

A goldfish grows just a tiny cluster of endochondral bones at the base of its fin. The rest of the fin is taken up by thin rays, which are made of an entirely different tissue called dermal bone. Dermal bone does not start out as cartilage and does not contain blood vessels.

These differences have long puzzled scientists. The fossil record shows that we share a common aquatic ancestor with ray-finned fish that lived some 430 million years ago. Four-limbed creatures with spines — known as tetrapods — had evolved by 360 million years ago and went on to colonize dry land.

Read more at http://mobile.nytimes.com/2016/08/18/science/from-fins-into-hands-scientists-discover-a-deep-evolutionary-link.html